Thin film capacitor and method of making same

ABSTRACT

A thin film capacitor having a counterelectrode of chromium overlaid by gold that completely buries the dielectric layer and protects same from any atmospheric or environmental contamination.

United States Patent Tierman 1 1 Jan. 25, 1972 I54] THINFILM CAPACITOR AND METHOD [56] References Cited OF MAKING SAME UNITED STATES PATENTS [72] Mel. Nmh Adams Mass' 3,257,592 6/1966 Maissel ..3 17/261 x [73] Assignee: Sprague Electric Company, North Adams, 3,403,439 10/1968 Bailey ..3l7/235 D Mass. Primary ExaminerE. A. Goldberg 1 d 1970 Attorney-Connolly and Hutz and Vincent H. Sweeney 21 Appl. No.: 89,206

' [57] ABSTRACT 52 us. (:1 .;...317/258, 317/101 A, 317/235 1), A thin film capacitor having tinunterelewode of chromium 3 l7/26 l 29/2512 overlaid by gold that completely buries the dielectric layer and 51] 1111.0. .1101; 1/02 protects Same m y atmospheric or environmental c n- [58) Field 6: Search ..317/258, 261, 101 A, 235 D, tamination.

14 Claims, 5 Drawing Figures tarnination and is more economical to fabricate.

1 THIN FILM CAPACITOR AND METHOD OF MAKING SAME BACKGROUND OF THE INVENTION I um-gold counterelectrode.

A problem in theprior art use of thin film capacitors has been the possibility of contamination by outside forces which leads. to a shortened life and undesirable interfering reactions.

' This contamination may come from any number of outside sources. One source would be from gases emitted from surrounding electrical components. Another source would be simple fingertip contamination that is, picked up on handling the thin filmcapacitors. Formed dielectric aluminum oxide thin film is particularly susceptible to breakdown when exposed to this contamination. I

A problem in the formation of thinfilm capacitors has been the formation of the counterelectrode. Two basic thin film capacitor types can be classified in terms of their counterelectrodes being nonadherent or adherent. In the nonadherent type a .unit comprises adielectric oxide of the thin film valvemetal electrode sandwiched between the electrode and a metal counterelectrode, usually gold because of its excellent electrical properties. This unit, however, is subject 'to mechanical breakdown because of poor adhesion of the counterelectrode and is also humidity sensitive because of moisture penetration to the interface of the dielectric and the poorly adherent counterelectrode/These deficiencies exclude the unit from many applications. The adherent type was designed to improve the mechanical properties of the unit by correcting the above deficiencies and in this sense succeeded. The counterelectrode was composed of a fthin" layer (over 200 A.) of

chromium, nichrome or titanium followed by the gold. The added metal lent adherence properties to the counterelectrode but had the undesired effect of degrading electrical properties, that is the breakdown voltage decreased and leakage current increased, thereby lowering yield. Another common problem with the batch fabrication of thin film capacitors is the complexity of oxidizingall of the valve-metal surface. at once, and the difficulty of maintaining 'unoxidized' contact areas to the underlying tantalum. Addition'ally,,the process of fabricating thin film capacitors necessitates many steps.

. Accordingly, it is an object of the present invention to provide a thin film capacitor ofthe adherent counterelectrode type which is completely protected from possible outside con- SUMMARY OF THE INVENTION The present invention is a thin film capacitor formed on a substrate and having a valve-metal oxide dielectric that is completely buried by the counterelectrode.

.In a preferred embodiment, a silicon substrate has a layer of silicon dioxide thermally formed thereon, and windows etched therein leaving configurations of exposed silicon with its oxide around it. Tantalum is deposited by a sputtering process and then photo defined with said tantalum film completely covering the exposed silicon substrate. The surface of the tantalum is anodized to form a tantalum pentoxide dielectric layer. A counterelectrode is then deposited which completely covers said oxide layer; this counterelectrode is composed of an extremely thin layer of chromium for adhesion purposes, a layer of gold followed by a gold layer plated thereon. The capacitor thus formed has a dielectric layer that is completely shielded from outside contamination. These units may be batch fabricated, and because the tantalum is in direct contact with the silicon substrate, the oxide dielectric layers on all of the units may be formed simultaneously. On another exposed section of the silicon substrate one can deposit another thin film. layer of chromium followed by gold to act' asan optional metallized contact area. The chromium layer used herein provides the adhesion necessary to improve the mechanical properties of the capacitor, and its extreme thinness makes'the properties of the overlying gold dominant, thereby maintaining the excellent electrical properties of that metal.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a completed capacitor unit; FIG. 2 is a partial view of a substrate showing six unseparated capacitors;

FIG. 3 is a perspective view of a completed capacitor unit having a beam lead thereon;

FIG. 4 is a cross section of the capacitor of FIG-.,.I, along line 4-4; and i I? FIG. 5 is a cross section of the capacitor of FIG. 3 having a beam lead thereon. Cross section is along line 5-5.-

DESCRIPTION OF THE PREFERRED EMBODIMENTS of the silicon substrate 13 there is a thin layer of chromium l2 followed by a layer of gold 10 forming an optional metallized contact area. The valve metal electrode and its dielectric oxide layer are buried and are not shown herein.

In FIG. 2 there is shown six unseparat'ed capacitor units out of approximately 1,000 that may be formed on a silicon wafer of 1 inches diameter. I

FIG. 3 shows the capacitor 'of FIG. 1 the only difference being that the gold layer 10 is extended outward to form a beam leaded counterelectrode.

FIG. 4 shows a cross section of the capacitor of FIG. I. A silicon substrate 13 has a formation of thermally grown oxide 11 with an etched out portion within the formed oxide configuration exposing said silicon substrate 13. A layer of tantalum 14 is deposited thereon and covers the etched out portion and extends upward onto the top of the oxide enclosure 11; the surface of the tantalum layer 14 is anodized giving tantalum pentoxide 15, the dielectric layer. A thin film of chromium '12 is then evaporated over the entire tantalum pentoxide layer 15, followed by a layer ofgold 10. The gold layer I0'is an evaporated film of gold followed by gold plating thereon.

The batch fabrication of these capacitors is carried out by first thermally growing silicon dioxide on a silicon wafer; then through masking and etching, small windows" are formed in the oxide, exposing the silicon through these windows"; the valve metal tantalum is then sputtered over the entire area thereon to a thickness of about 3,000 A. and little islands of tantalum are formed by etching, said islands covering at least all of the then exposed silicon substrate. These tantalum islands are anodized forming tantalum pentoxide. The anodization is done with phosphoric acid or some other suitable formation electrolyte at an anodization voltage of around v. A very thin layer of chromium is then evaporated over this entire surface of the silicon substrate followed by the evaporation of a thin layer of gold. A photoresist mask is then placed in the channels and the gold layer is then gold plated on the island areas. After removal of the masking, the thin film layer of gold and chromium that is beyond the plated gold islands is then etched away leaving said islands surrounded by the silicon dioxide. A grid pattern is then formed and channels are etched in the silicon dioxide between each unit. The front of the wafer is then wax mounted on a ceramic disc for protection while the wafer is thinned on the bottom by etching the silicon away. Silicon is not a highly conductive material, so it is thinned to form a sheet not more than 2 mils thick and to reduce its resistivity. This makes the silicon conductive enough to suit the purpose of the present invention-that is, conductive enough to allow conductivity through this substrate to the tantalum electrode. A highly doped silicon can also be used to reduce the resistivity therein. At this point, thin film layers of chromium and gold may optionally be evaporated on the bottom thereon for contact areas and the wafer is removed from the ceramic disc, and any residual wax is then removed with a solvent. The wafer is then scribed with a diamond scriber from the front and the units are broken and the capacitors separated.

Over 1,000 capacitors can be produced on one l%-inches diameter silicon wafer. Each capacitor is approximately 0.020 0.020X 0.004. The capacitanceof these units is approximately l pf. and the dissipation factor is 0.2 percent. These units have 2 l0 a. leakage at 125 v. 50 WVDC rating 190 v. DC formation). The dielectric so formed is approximately 2,800 A. thick, resulting in a decrease of approximately 1,400 A. of the tantalum film.

A capacitor having a counterelectrode of gold that extends out to form a beam leaded contact is shown inFlG. 5. This figure is basically the same as described in FIG. 4 except that when the chromium film l2 and gold film are evaporated on the surface and followed by the gold plate, the gold plate extends beyond the island and onto the silicon substrate area beyond the oxide enclosure. When the silicon substrate 13 is thinned, as previously described herein, the silicon directly underthe extending gold plate is then etched completely away exposing the cantilevered portion of gold plate. This unit also completely buries the dielectric but the counterelectrode has a beam leaded contact'area. The rest of the process, including the optional metallization of the electrode to the bottom of the silicon layer, is carried out in the same manner as was previously described herein.

The above embodiment describes the use of tantalum as the valve metal; other valve metals that can be used include aluminum, hafnium, niobium, titanium and zirconium.

Also, the preferred embodiment speaks of a silicon substrate, but other conductive substrates can be used with similar but less desirable results. Some of these include germanium and aluminum another valve metals. In these cases it .may be more desirable to form a nitride configuration in situ on the conductive substrate instead of its oxide. Any wellknown passivating technique may be employed to form the nitride configuration.

While the preferred embodiment speaks of forming individual capacitor units, it should be understood that one may form such a capacitor on a silicon substrate that is part of an integrated circuit. Other electrical components that may appear thereon include transistors, resistors, diodes and so forth.

Since it is obvious that many changes and-modifications can be made in the above described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not limited to said details except as set forth in the appended claims.

What is claimed is:

l. A thin film capacitor comprising a conductive substrate;

a configuration of in situ formed dielectric compound of said substrate on a surface of said substrate and having an area of the substrate exposed within the configuration;

a thin film deposit of a valve-metal electrode over at least all of said surface exposed within said configuration, said film being in electrical contact with the substrate;

a dielectric layer of the oxide of said valve-metal electrode formed on'the surface of said film that faces away from the conductive substrate; and

a counterelectrode completely covering oxide layer, and extending beyond same.

2. The capacitor of claim 1 wherein the conductive substrate is silicon.

3. The capacitor of claim 2 having a metal contact attached to an exposed surface of the silicon substrate.

4. The capacitor of claim 1 wherein the valve metal is tantalum.

5. The capacitor of claim 1 wherein the valve metal is aluminum.

6. The capacitor of claim 1 wherein said counterelectrode is a thin film of chromium covered by a thin film of gold that is plated with gold.

7. The capacitor of claim 1 having a beam lead extending from said counterelectrode.

8. The capacitor of claim 1 wherein said counterelectrode is a film of a nickel-chromium alloy covered by a layer of gold which is plated with gold.

9. The capacitor of claim 1 wherein said substrate is silicon, said dielectric compound is silicon dioxide, said valve metal is tantalum, said dielectric layer is tantalum pentoxide, and said counterelectrode is a chromium film covered by a gold film plated with gold.

10. A method of fabricating a thin film capacitor including the steps of:

providing a conductive silicon substrate with a configuration of in situ formed oxide around the top thereof;

said valve-metal applying a thin film of a valve metal onto at leastthe entire substrate surface within said configuration;

forming a layer of the oxide of said valve metal on the ex- 14. The method of claim 13 wherein the subdividing is effected in a manner that leaves a beam lead projecting from the counterelectrode of a piece. 

2. The capacitor of claim 1 wherein the conductive substrate is silicon.
 3. The capacitor of claim 2 having a metal contact attached to an exposed surface of the silicon substrate.
 4. The capacitor of claim 1 wherein the valve metal is tantalum.
 5. The capacitor of claim 1 wherein the valve metal is aluminum.
 6. The capacitor of claim 1 wherein said counterelectrode is a thin film of chromium covered by a thin film of gold that is plated with gold.
 7. The capacitor of claim 1 having a beam lead extending from said counterelectrode.
 8. The capacitor of claim 1 wherein said counterelectrode is a film of a nickel-chromium alloy covered by a layer of gold which is plated with gold.
 9. The capacitor of claim 1 wherein said substrate is silicon, said dielectric compound is silicon dioxide, said valve metal is tantalum, said dielectric layer is tantalum pentoxide, and said counterelectrode is a chromium film covered by a gold film plated with gold.
 10. A method of fabricating a thin film capacitor including the steps of: providing a conductive silicon substrate with a configuration of in situ formed oxide around the top thereof; applying a thin film of a valve metal onto at least the entire substrate surface within said configuration; forming a layer of the oxide of said valve metal on the exposed surface of said valve metal film; and burying said layer under a counterelectrode.
 11. The method of claim 10 in which the substrate is relatively thick and after the fabricating is reduced in thickness to form a sheet not more than about 2 mils thick.
 12. The method of claim 10 having a metal contact attached to a surface of said substrate.
 13. The method of claim 10 wherein the substrate is subdivided into individual pieces, each comprising at least one of said configurations.
 14. The method of claim 13 wherein the subdividing is effected in a manner that leaves a beam lead projecting from the counterelectrode of a piece. 